Despite advances in treatment, severe mental illness (SMI) remains common, chronic and difficult to treat. SMI is defined as having at least one mental disorder that lasts for 12-months and leads to substantial life interference. Sleep and circadian dysfunctions are among the most prominent correlates of SMI, yet have been minimally studied in ways that reflect the complexity of the sleep problems experienced by people with SMI. In SMI, sleep and circadian dysfunction undermines affect regulation, cognitive function and physical health, predicts onset and worsening of symptoms and is often chronic even with evidence-based SMI treatment. Prior treatment studies have been disorder-focused-they have treated a specific sleep problem (e.g., insomnia) in a specific diagnostic group (e.g., depression). However, real life sleep and circadian problems are not so neatly categorized, particularly in SMI where features of insomnia overlap with hypersomnia, delayed sleep phase and irregular sleep-wake schedules. Accordingly, we aim to test the hypothesis that a Trans diagnostic Intervention for Sleep and Circadian Dysfunction (TranS-C) will improve functional impairment, disorder- focused symptoms and sleep and circadian functioning. We will recruit participants across DSM diagnoses and across common sleep and circadian problems. The elements of TranS-C are efficacious across SMI in research settings with research-based providers. The next step is to test TranS-C in community settings with community-based providers. Accordingly, we propose to conduct an 'efficacy in the real world' randomized controlled trial within Alameda County Behavioral Health Care Services (ACBHCS), the CMHC for Alameda County. Alameda County is the most ethnically diverse county in the Bay Area. We will recruit 120 adults diagnosed with SMI and sleep and circadian dysfunction within ACBHCS. Individuals will be randomly allocated to TranS-C (n = 60) or 6-months of Usual Care followed by Delayed Treatment with TranS-C (UC- DT; n = 60). TranS-C is modularized and delivered across eight 50-minute, weekly, individual sessions. All participants will be assessed before, immediately following treatment and again 6 months later. Mediation analyses will examine whether improved sleep and circadian rhythm functioning mediate improvements in primary outcomes. Moderation analyses will examine if intervention efficacy is related to previously reported risk factors including demographics, symptom severity, medications, site, and season of treatment as well as psychiatric and medical comorbidity. This application is responsive to NIMH priorities focusing on implementing effective services in the community as well as treatments that are personalized, generalizable and not constrained by diagnostic criteria. RDoC is addressed by recruiting participants across diagnoses and defining sleep and circadian dysfunction broadly. The 'sleep-wake' and 'biological rhythms' constructs from the Arousal and Modulatory Systems domain are intervention targets and outcome measures. Our long-term objective is to establish the potential for widespread dissemination of TranS-C.